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Old 10th October 2007, 08:47 PM   #1
mgalvez is offline mgalvez  United States
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Default First Timer needs help

Hello, this is my first post here. I've been wanting to get into tube amplifiers for a long while now. I just picked up this vintage tube amp off E-bay and was wondering if anyone could help me identify it. I was told it is from an RCA console and set up for 8ohm speakers. It has two RCA electron tubes marked "6AQ5A". One RCA electron tube marked "12AX7A". One RCA electron tube marked "5Y3GT". It also has some wires sticking out visible in the pics and I have no idea what these are. I want to get this thing sounding good, but I have a long way to go. Any help would be greatly appreciated. Also, does anyone have a good speaker recommendation? I was looking at building some Dayton III speakerbuilder.net.


Click the image to open in full size.]Tube Amp[/URL]


Click the image to open in full size.]Underside[/URL]

Click the image to open in full size.]Tubes[/URL]


Click the image to open in full size.
]Mystery Wires[/URL]


Click the image to open in full size.
]Mystery Wires 2[/URL]
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Old 10th October 2007, 11:23 PM   #3
Sherman is offline Sherman  United States
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The 6AQ5As are 7 pin pentode power tubes. The 12AX7a is the input stage/voltage amp. The 5Y3GT is the rectifier tube.

First up, before turning it on replace all the capacitors. Those appear to be the originals and are probably leaky/dried/dead. I would also replace the resistors as they have probably drifted quite a bit.

The amp is a single-ended stereo. With the 6AQ5A tubes in pentode mode it might make 4 to 5 watts per channel. In triode mode maybe 1 1/2 watts per channel.

Your choice of speakers depends on the size of your listening room, the distance from the speakers, the type of music you listen to and how loud you'd like it to be. In triode mode you would need very efficient speakers to get very loud with 1.5 watts.

The mystery wires appear to be the wires that went to the speakers in the console, though it would be a good idea to trace out the schematic.

Others with more expertise will undoubtedly be able to offer more info.
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Old 11th October 2007, 12:00 AM   #4
mgalvez is offline mgalvez  United States
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Sherman, thank you very much. I guess my first task is to get a schematic. I will test the caps to see if they are within spec. Is there any way to test tubes? Should I just get new ones? Any other recommendations or info would be greatly appreciated.
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Old 11th October 2007, 12:00 AM   #5
rman is offline rman  Canada
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Hi mgalvez.

My first suggestion would be to chekout the thread called
online tube learning for newbies. How is you basic electronics knowledge? If you have the basics of dc, ac, ohms law, what capacitors do and so forth, tube amplifiers are quite simple especially a single ended one like yours. checkout sites like
fun with tubes and boozhound labratories.

When I built my first amp, I was concerned that it wouldn't have
enough power to drive my speakers which are rated at 90db
for 1 watt. That amp puts out about 1.2 watts per channel
and is plenty loud, especialy since I have moved into a small
apartment. 90db/watt is fairly average.



Fun With Tubes
boozhoundlabs.com/howto
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Old 11th October 2007, 02:32 AM   #6
mgalvez is offline mgalvez  United States
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Thanks rman, I have a lot to learn. Somebody found the schematic for the amp. Hopefully this will get my project moving foreward.

http://img165.imageshack.us/img165/9...35smallbi1.jpg

So will I be able to hook up a low level input from a source such as a cd player without any modifications? Can I make this amp sound good with speakers such as these?

http://www.speakerbuilder.net/web_fi...D3/dayton3.htm

Thanks again for help.
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Old 11th October 2007, 05:53 AM   #7
rman is offline rman  Canada
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It is not clear from the diagram what impeadeance speakers
this amp was built for. With tube amps this is important because
the circuit is made for aspecific load. A lower impeadeance load
will get more power but will probably cause the output tube
create quite a bit of distortion. I would sugest to find a very cheap
pair of 8 ohm speakers maybe from a thrift shop just to see if the thing actually works. make sure there is a fuse between the
power plug and the amp before pluging it in. Also make sure
speakers are connected first as well.

It is hard to see from the pictures exactly witch wires connect to
the speakers and the diagram is too small to make out the writing. The two transformers sticking out each end are the output. Trace the wires following the diagram. Each output will
have one wire to a tube and one to the power supply this is the primary side. The speakers connect to the secondary side. You can plug your cd right in to the rca jacks. It might sound quite nice
but I wouldn't expect too much. Just use it as a learning tool.

Have fun but be carfull. do not put your finders inside while it is plugged in or even for a while after you unplug it because the
large capacitor in the power supply can store power for a long time!

Good luck!
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Old 11th October 2007, 05:53 AM   #8
rman is offline rman  Canada
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one more thoght, It looks like this is a dedeicated phono amp for ceramic cartriges. ceramics have a high output so it should work with a cd player... but. It also has a built in equalisation network that strongly boosts the bass and cut the treble because this is needed for playing all vinyl records. maybe if you turn the bass way down and the treble way up it will sound not to bad.

The two resistors and two capacitors just to the left of the loudness control on the diagram form this network. Perhaps
they could be bypased to make this more suitable for cd, tuner,
or other such input.

Cheers
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Old 11th October 2007, 03:57 PM   #9
mgalvez is offline mgalvez  United States
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Rman, is this amp worth modding? I just want a an amp with a clean sound. My listening area is small, so I don't think I need more than a few watts. I am going to start testing today.
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Old 12th October 2007, 12:22 AM   #10
rman is offline rman  Canada
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Hi mgalvez.

I doubt that your little amp will ever really be a truly hi-fi amplifier.
Maybe by getting rid of that phono eq network, and puting in a decent pair of output transformers, it might be a decent performer.

If you just want a great sounding very simple power amp and don't need alot of wattage, my best suggestion would be to build
a copy of Gary Kaufman's direct coupled 6em7 amp. You can find
the circuit and pictures of it here.

gary's DC 6EM7

It should work fine with those speakers you are thinking of.
They are 4 omh speakers so you need out put transformers
that will match 5000 omhs to 4 instead of 8. I built one with
Edcore transformers the small single ended ones, and I really
It. It is very simple and the tubes are cheap. You just have to pick good parts and build it carefuly and it will turn out very well.


Good luck.
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